The present invention relates to a method and a device for manufacturing a GaN semiconductor crystal including gallium (Ga) and nitrogen (N).
Recently, a quality thin film of a GaN crystal can be obtained by using the metal organic vapor phase epitaxy (MOVPE) for growing a GaN semiconductor crystal on a substrate of sapphire (Al.sub.2 O.sub.3). As a result, a light emitting device with a short wavelength has been realized. Under such circumstances, a blue light emitting diode (LED) has become capable of emitting light in a wavelength region ranging between blue and green, and has started to be practically used, together with a red LED, in a multicolored LED display device and a traffic signal. Furthermore, realization of a blue laser diode by using a GaN crystal has increased future possibility that quality image data can be recorded in a compact disk recording medium.
Also, in accordance with the development of the GaN crystal growth technique, a high frequency electronic device having both a high breakdown voltage and an environmental resistance has become realized. Specifically, in high frequency electronic devices using a GaAs or InP semiconductor, those used at large power where the breakdown voltage is insufficient or used in the case where an additional mechanism for heat radiation, cooling or the like is conventionally required have been regarded to become applicable to simplification and compactness of a device set by eliminating such an additional mechanism.
For example, first paper "Jpn. J. Appl. Phys. 30, L1705 (1991)" describes growth of a GaN semiconductor crystal on a substrate of sapphire for realizing a light emitting device with a short wavelength. Also, second paper "Appl. Phys. Lett. 62, 702 (1993)" describes growth of a GaN semiconductor crystal on a substrate of silicon carbide (SiC) for realizing an electronic device with a high breakdown voltage.
As is described in these papers, in the case where different materials are used in a substrate and a crystal layer grown on the substrate by the MOVPE or a molecular beam epitaxy (MBE), in general, a buffer layer is grown on the substrate first and the desired crystal layer is grown on this buffer layer in order to relax discontinuity in the interface between the substrate and the crystal layer.
The growth of a GaN crystal on the substrate of sapphire described in the first paper, however, has the following several problems:
Sapphire and GaN both have a crystal structure of the hexagonal system. Although a GaN crystal grown at a comparatively low temperature of approximately 500.degree. C. is used as the buffer layer, the lattice constants of sapphire and GaN are largely different from each other, and specifically, sapphire has the lattice constant of 2.74 .ANG. and GaN has the lattice constant of 3.189 .ANG.. Therefore, there is lattice mismatch of approximately 14% between them. Such a large difference in the lattice constant causes not only large strain in the vicinity of the interface between the sapphire and the GaN crystal but also dangling bonds, resulting in causing a large number of dislocations in the GaN crystal. Accordingly, even an LED practically used at present has a defect density of 1.times.10.sup.10 cm or more, and these defects lead to a large number of non-light emitting areas within the diode. Also, since light scattering and the like is caused by these defects, the luminous efficiency of the diode is largely degraded. Furthermore, the defects can be increased or expanded during the operation of the diode, so that the performance of the diode can be degraded and that the diode itself can be ultimately damaged. In this manner, such a defect is a factor in shortening the lifetime and degrading the reliability of the diode.
Moreover, although the substrate of sapphire has advantages of a low cost and stable quality, it is disadvantageous in the manufacture of devices because it cannot be cleaved and hence cannot be divided for isolation. In addition, the substrate of sapphire itself has an insulating property, and hence, it is impossible to adopt a device structure in which electrodes are respectively formed on a device forming surface and the other surface of the substrate. Accordingly, the manufacturing process can be disadvantageously complicated.
On the other hand, the growth of a GaN crystal on a substrate of SiC disclosed in the second paper has the following several problems:
Since a SiC crystal has the hexagonal system and a lattice mismatch ratio with GaN is as small as 3%, such a substrate is less affected to be degraded in the crystallinity by the lattice mismatch as compared with sapphire. Also, since SiC itself is conductive, electrodes can be formed on a device forming surface and the other surface of the substrate of SiC. Thus, the substrate of SiC can be regarded superior to the substrate of sapphire.
However, a generally available SiC substrate includes 1.times.10.sup.5 cm.sup.-2 or more etch pits, and these etch pits can disadvantageously cause a large number of defects in an epitaxial crystal layer grown on the substrate. Therefore, it is difficult to increase the yield of the epitaxial crystal layer, which makes efficient manufacture very difficult.
Moreover, SiC includes, in addition to the etch pits, a defect designated as a micropipe. The density of the micropipes is generally approximately 1.times.10.sup.2 cm.sup.-2, but each micropipe has a sectional area as large as 0.1 mm.sup.2, which further degrades the yield of the epitaxial crystal layer. In addition, SiC is difficult to manufacture, and hence is very expensive. Therefore, it is difficult to decrease the manufacturing cost of a device using SiC as a substrate.